package algorithm.dp;

// 第 N 个泰波那契数
public class LC1137 {
    // 空间优化
    public int tribonacci(int n) {
        if (n == 0) {
            return 0;
        }

        if (n == 1 || n == 2) {
            return 1;
        }

        int a = 0, b = 1, c = 1, d = 0;

        for (int i = 3; i <= n; i++) {
            d = a + b + c;
            a = b;
            b = c;
            c = d;
        }
        return d;
    }

    public int tribonacci0(int n) {
        // 0. 处理边界情况
        if (n == 0) {
            return 0;
        }

        if (n == 1 || n == 2) {
            return 1;
        }

        // 1. 创建 algorithm.dp 表
        int[] dp = new int[n + 1];

        // 2. 初始化
        dp[0] = 0; dp[1] = 1; dp[2] = 1;

        // 3. 填表
        for (int i = 3; i <= n; i++) {
            dp[i] = dp[i - 1] + dp[i - 2] + dp[i - 3];
        }

        // 4. 返回值
        return dp[n];
    }
}
